In the automotive field in which improvement in fuel consumption with weight reduction of car bodies is becoming an important issue, thinning of high-strength steel sheets applied to automobile parts is advanced, and use of steel sheets with a TS of 980 MPa or more is advanced.
High-strength steel sheets used for automobile structural members and reinforcing members are required to have excellent formability and impact energy absorption properties. In forming parts having a complicated shape, the steel sheets are required to be not only excellent in respective characteristics such as excellent elongation and stretch flangeability (hole expansion formability), but also excellent in both characteristics and, particularly, an elongation of 20% or more is required to form parts that are required to be bent. Also, to improve impact energy absorption property, it is effective to enhance the yield ratio, and impact energy can be effectively absorbed even with a small strain.
A dual phase steel sheet (DP steel sheet) having a ferrite-martensite structure has been known as a high-strength thin steel sheet having both formability and high strength. However, the DP steel sheet has excellent elongation with strength, but cracks easily occur due to the concentration of stress in a ferrite-martensite interface, thereby causing the disadvantage of low bendability and hole expansion formability. Therefore, for example, Japanese Patent No. 4925611 discloses a DP steel sheet with excellent elongation and bendability imparted by controlling the crystal grain diameter, volume fraction, and nano-hardness of ferrite. Also, a TRIP steel sheet is known as a steel sheet having both high strength and excellent ductility. TRIP steel sheet has a steel sheet structure containing retained austenite, and working deformation at a temperature equal to or higher than the martensite transformation start temperature causes stress-induced transformation of the retained austenite to martensite, producing large elongation. However, the TRIP steel sheet causes transformation of the retained austenite to martensite during punching and thus cracks occur at an interface with ferrite, thereby causing the disadvantage of low hole expansion formability. Therefore, Japanese Patent No. 4716358 discloses a TRIP steel sheet containing bainitic ferrite.
However, in general, moving dislocation is introduced in ferrite in a DP steel sheet during martensite transformation and, thus, the yield ratio is decreased, thereby decreasing the impact energy absorption property. Also, the steel sheet of Japanese Patent No. 4925611 has insufficient elongation with a tensile strength (TS) of 980 MPa or more, and satisfactory formability cannot be secured. Further, in having a tensile strength (TS) of 980 MPa or more, the steel sheet of Japanese Patent No. 4716358 utilizing retained austenite has a yield ratio (YR) of less than 75% and, thus, has the low impact energy absorption property. Therefore, it is difficult for a high-strength steel sheet having a tensile strength (TS) of 980 MPa or more to secure elongation and hole expansion formability that achieves excellent press formability, while excellent impact energy absorption property is maintained. In an actual situation, including other steel sheets, there have not been developed steel sheets that satisfy these characteristics (yield ratio, strength tensile strength, elongation, and hole expansion formability).
Accordingly, it could be helpful to provide a high-strength cold-rolled steel sheet having excellent elongation and hole expansion formability and a high yield ratio and a method of producing the steel sheet.